1. Field of the Invention
The present invention relates to a fixing apparatus used with an image forming apparatus such as a copying machine, a printer and the like.
2. Related Background Art
In the past, in fixing apparatuses used with an image forming apparatus such as a copying machine, a laser beam printer and the like, toner consisting of resin, magnetic material, coloring agent and the like electrostatically transferred onto a recording sheet at an image forming portion was fixed to the recording sheet by heating and fusing the toner.
FIG. 4 shows an example of a conventional fixing apparatus. This fixing apparatus comprises an upper heat roller 51 and a lower pressure roller 52. The heat roller 51 includes therein a main heater 53a and a sub-heater 53b. The main heater 53a is always used in a fixing operation, but, the sub-heater 53b is used only in a start-up operation to shorten a wait up time. The recording sheet P is passed through a fixing nip between the heat roller 51 and the pressure roller 52. The toner 54 transferred from a photosensitive member to the recording sheet P at the image forming portion is electrostatically held on the recording sheet in a non-fixed condition. When the recording sheet P passes through the fixing nip, the toner 54 is heated by the heat roller 51 directly contacting with the toner and is pressurized by the pressure roller 52 urging the recording sheet P against the heat roller 51, with the result that the toner 54 is fused and fixed to the recording sheet by heat and pressure.
If the toner 54 is fixed at a temperature lower than a temperature at which the resin and the like constituting the toner is softened or melt, low temperature offset will occur. That is to say, in the fixing operation, there are optimum upper temperature limit and lower temperature limit. Thus, only when the fixing temperature is controlled within a range between the upper and lower limits, the good fixing can be achieved. The control of the fixing temperature is effected on the basis of a surface temperature of the heat roller 51 detected by a thermistor 55.
However, in the fixing apparatus having the above-mentioned construction, if a copying speed is increased in a continuous copy mode (continuous image forming mode), as the number of copies is increased, a heat amount greater than that supplied from the heat roller 51 is absorbed by the toner 54 and the recording sheet P, with the result that, even when the heater is fully activated, the temperature detected by the thermistor 55 and accordingly the fixing temperature will be decreased. To avoid this, electric power supplied to the main heater 53a may be increased. However, in this case, power consumption require by the copying machine (image forming apparatus) is increased, with the result that such image forming apparatus needs more power than is available in typical residences, thereby limiting the installation locations for the image forming apparatus. Further, in recent years, since the improvement in productivity of the copying machines has been required to provide faster copying machines, the percentage of power consumption of the fixing apparatus regarding the total power consumption has been increased. Thus, the specification of the image forming apparatus has been become severe to use the image forming apparatus in the general homes.
To avoid this, there has been proposed a technique in which, when the temperature of the heat roller 51 reaches the lower fixing temperature limit during the continuous copying mode, the copying operation is temporarily stopped and the fixing operation is waiting until the temperature of the heat roller 51 is restored to the optimum temperature.
Alternatively, when the temperature of the heat roller reaches a temperature slightly higher than the lower fixing temperature limit the copy frequency may be extended to decrease the reduction ratio of the fixing temperature (reduction of temperature per each copy), thereby extending the time period reaching the lower fixing temperature limit. In this method for extending the copy frequency, there has been proposed a technique wherein a fixing speed is determined on the basis of the reduction ratio of the fixing temperature and the number of remaining copy sheets to avoid "copy stop". That is to say, during the continuous copying operation, if the fixing temperature is decreased, the copy frequency is suitably extended so that the time period between the copy regarding a certain recording sheet P and the copy regarding a next recording sheet P is set to restore the temperature of the heat roller 51 completely.
However, in the above-mentioned conventional technique, there arose a problem that, in the continuous copy mode, the total copying time for copying all of the sheets which were set as the number of copies is increased. That is to say, since this technique is not a system for controlling the entire copying time to the minimum but is a system for preventing the poor fixing during the continuous copy mode, the total copying time becomes greater than that in a copying machine having the latest copying speed. Thus, the productivity of the copying machine cannot be improved.